Electric dust collector

ABSTRACT

An electric dust collector that does not need an exhaust fan or special equipment for preventing dust from flying off. An incinerator is formed on a building, and a smokestack is raised so that it is integrated with the wall of the incinerator. The smokestack is formed from metallic sheeting and lined with refractories. A beam is installed over the smokestack. The beam is electrically insulated using insulatores. A discharge electrode is suspended form the beam at the center of the smokestack. The discharge electrode has many needle-like discharge pins almost in its lower half part. The negative pole of a DC high-voltage power supply is connected with the discharge electrode, while the positive pole is grounded and connected with the metallic sheeting constituting the smokestack.

This application is the National Stage Application filed under 35 USC371 of International Application No. PCT/JP96/02242 filed on Aug. 7,1996.

TECHNICAL FIELD

The present invention relates to an electric dust collector also servingas an exhaust pipe lined with refractories.

BACKGROUND ART

Conventional electric dust collectors, used to remove dust from exhaustfumes and exhaust gas from smelting furnaces (steel converters, cupolas,etc.), heat treatment ovens (annealing ovens etc.), ceramics ovens(cement kilns etc.), incinerators, drying ovens, and heat engines, arearranged so that exhaust fumes or exhaust gas passes between manyclosely spaced dust collecting electrodes. This causes a heavy pressureloss. To prevent an electric dust collector from being exposed to hightemperatures, a cooler or the like, also serving as a heat recovery heatexchanger or a gas neutralizer, is normally installed immediatelyupstream of the electric dust collector, thus increasing pressure loss.Due to pressure loss thus caused, only aeration is not sufficient toemit exhaust fumes or exhaust gas, and accordingly, an exhaust fan isessential for the emission. Because of their exposure to corrosive gasand heat, exhaust fans fail so frequently that they require laboriousmaintenance and inspection.

In addition, conventional electric dust collectors have the followingproblems:

(i) Hammering performed to remove dust deposits from collectingelectrodes dust to fly off.

(ii) A high gas flow rate aerodynamically causes dust to fly off.

(iii) Dust flies off due to reverse ionization occurring at a high dustelectric resistance of more than 10¹² Ω·cm.

(iv) Dust abnormally flies off due to a low dust electric resistance ofless than 10⁴ Ω·cm.

These problems reduce the rate of dust collection. To avoid theproblems, conventional electric dust collectors require the followingtroublesome countermeasures:

(i) Exhaust fumes and exhaust gas humidity adjustment

(ii) Dust collection at a high temperature around 360° C.

(iii) Wet-type dust collection

(iv) Pulse charging

It is an object of the present invention to provide an electric dustcollector that needs no exhaust fan or special equipment dedicated toprevent dust from flying off.

DISCLOSURE OF THE INVENTION

An exhaust pipe (smokestack) according to the present invention,emitting into the air exhaust fumes from a burning chamber of anincinerator and so on, is made of steel sheeting and lined withrefractories, since it is exposed to elevated temperatures due toexhaust fumes. The refractories are preferably castable refractories(refractive concrete composed mainly of SiO₂ and Al₂ O₃) and the like.

The refractories are insulators at normal temperature, which conductlittle electric current. During operation, the temperature at thesurface of the refractories is around 800° C., and that nearheat-resistant anchors preventing the refractories from falling offreaches 400 to 500° C. At such high temperatures, ordinary refractorieslose their performance of electric insulation as an ordinary insulatingmaterial does. FIG. 1 shows the temperature dependence of a castablerefractory.

Corona discharge requires high voltages. Since corona discharge involvesa small current, however, the dust collecting electrode does not alwaysneed to be as conductive as metal. The electrode is only required tohave enough conductivity to discharge collected dust.

According to the present invention, a discharge electrode is suspendedat the center of an exhaust pipe, and a high DC voltage is appliedbetween the discharge electrode and the external steel sheeting of theexhaust pipe (the steel sheeting and the discharge electrode havepositive and negative polarity, respectively, as in general electricdust collectors).

The collector arranged as described above has a considerably longdischarge gap both because the discharge electrode is at the center ofthe exhaust pipe (smokestack) and because the internal surface of thepipe is a dust collecting electrode. No literature describes in detailcorona discharge occurring in such a long gap at high temperatures.Using a small dust collector with a short discharge gap, the inventorsmade sure that an electric current with two orders or more of magnitudelarger flows in a gas at an atmospheric pressure and a temperature of500 to 700° C. than in a gas at normal temperature when the same voltageis applied. FIG. 2 shows some of the data obtained with the small dustcollector. If the discharge gap is long as is the case with thecollector, many discharge pins installed in the direction of dischargeelectrode length allow satisfactory corona discharge to be caused by anordinary DC high-voltage power supply applying a voltage that is nothigh relative to the long discharge gap.

For spark discharge, the spark initiating voltage is said to be directlyproportional to the product of the discharge gap and the gas density.Even a long discharge gap as is the case with the collector thereforeallows the discharge voltage to be high, so that the difference betweenspark initiating voltage and operating voltage can be set high. Thussafe collector operation can be performed which is free ofshort-circuiting due to spark discharge.

When corona discharge occurs, gas molecule ionization progresses toproduce many cations and anions. Then the cations are immediatelyneutralized by the discharge electrode, and the anions and electrons runtoward the dust collecting electrode. When burnt gas passes through anelectric field produced by corona discharge, particles (dust) in the gasare instantaneously charged by collisions of ions and electrons.

The charged dust is attracted to the dust collecting electrode under theeffect of the electric field between discharge electrode and dustcollecting electrode, that is, the refractories. According to thepresent invention, the distance between discharge electrode and dustcollecting electrode is long, and practical limitations are placed onthe voltage of a high-voltage power supply. Thus a strong electric fieldis difficult to produce, causing charged particles to be weaklyattracted to the dust collecting electrode. However, both the dischargeand dust collecting electrodes extending along the flow of exhaustfumes, coupled with a low flow rate of exhaust fumes due to a largeexhaust pipe cross section, prolong the time required for chargedparticles to pass through an electric field. This allows a satisfactorydust collecting effect to be exercised even with a weak electric field.

Dust deposited on the dust collecting electrode, the internal surface ofthe exhaust pipe, peels and falls off under the action of its ownweight. For conventional dry-type electric dust collectors, dustdeposited on the dust collecting electrode has been removed byperiodically hammering or vibrating the electrode. Dust strongly adheresto the dust collecting electrode because dust particles conglomeratewith the help of water. According to the present invention, dust isexposed to high temperatures, thus removing water therefrom, so thatdust particles bond together into large conglomerations due to anelectrode conglomeration effect and fall off under the action of theirown weight without being deposited on the dust collecting electrode toform a thick layer.

The present invention allows an exhaust pipe to have so large a crosssection that the flow rate of exhaust fumes can satisfactorily bereduced, thus preventing dust collected from flying off due toaerodynamic effects.

It is natural that the present invention can also be applied to a smalldust collector having an exhaust pipe with little exhaust fumes and asmall cross section. As can be seen from the data in FIG. 2, a smalldust collector has a relatively short discharge gap, and therefractories in the collector reach high temperatures. Accordingly, itsoperating voltage can be reduced, compared with conventional electricdust collectors. This allows the power supply and insulating means to besimplified.

It is well known that installing tipped discharge pins with a largeradius of curvature to the discharge electrode promotes coronadischarge. When the present invention is applied to a dust collectorwith a long discharge gap, the operating voltage can be reduced byinstalling many discharge pins to the discharge electrode.

When the present invention is applied to a dust collector with a shortdischarge gap, choosing the number of discharge pins and their shape asdesired allows the operating voltage to be changed.

The exhaust pipe may be formed independently of the burning chamber. Theexhaust pipe can be installed immediately under the burning chamber tohelp remove dust deposits peeling off the internal surface of theexhaust pipe by taking the deposits together with ash on the burningchamber floor out of the collector. This eliminates the need for ahopper receiving dust peels. The flow rate of exhaust fumes in theexhaust pipe is so low that dust peeling off the internal surface of theexhaust pipe slowly falls along the wall.

For conventional dry-type electric dust collectors, if the electricresistance of dust is high, it may undergo reverse ionization, thusflying off from the dust collecting electrode. The higher thetemperature, the lower the electric resistance of dust. According to thepresent invention, however, dust collecting is performed directly fromexhaust fumes of high temperature in the exhaust pipe. Therefore, theelectric resistance of dust is kept so low that dust hardly flies offdue to reverse ionization. In addition, since the dust collectingelectrode, or refractories, of a dust collector according to the presentinvention is not completely conductive, the charge on the dustcollecting electrode is slowly neutralized in a long time even if theelectric resistance of dust is low. Thus dust is further inhibited fromflying off due to a low dust resistivity.

The present invention has advantages below.

A first electric dust collector according to the present invention isadapted to support a discharge electrode almost in the axis of anexhaust pipe and to apply a DC voltage between the discharge electrodeand the metal sheeting constituting the exhaust pipe. The exhaust pipeis filled with high-temperature exhaust fumes. Thus the first electricdust collector can attract dust to the refractories inside the exhaustpipe, the refractories becoming conductive at elevated temperatures, bycharging the dust using corona discharge caused between the dischargeelectrode and the internal surface of the exhaust pipe at a lowervoltage than when the exhaust pipe is filled with low-temperatureexhaust fumes. The first electric dust collector is simple and requiresonly a low operating cost. The collector is also easy to maintainbecause it does not need an exhaust fan or special equipment preventingdust from flying off.

A second electric dust collector according to the present invention hasdischarge pins around a discharge electrode. Even if the second dustcollector has a long discharge gap, the discharge pins, coupled withease of corona discharge at high temperatures, allow the second dustcollector to be satisfactorily operated using an ordinary DChigh-voltage power supply producing a voltage which is not significantlyhigh.

If the discharge gap is not considerably long, the number of dischargepins and their shape can be chosen to vary the operating voltage, sothat the second dust collector can be operated according to the limitson the length of the discharge electrode and dust collecting electrodemeasured along the direction of the flow of exhaust fumes, the powersupply, insulation, and exhaust fume temperature.

A third electric dust collector according to the present invention hasan exhaust pipe which is aligned with a burning chamber and integratedwith the wall of the burning chamber. Being so arranged, the third dustcollector allows dust caught on a dust collecting electrode to directlyfall into the burning chamber after it peels off. This eliminates theneed for a hopper receiving dust and allows dust fallen into the burningchamber to be conveniently discarded together with ash in the burningchamber.

A fourth electric dust collector according to the present invention hasa beam installed over a structure independent of an exhaust pipe, fromwhich beam a discharge electrode is suspended into the exhaust pipe.Although simple, the beam has an advantage of supporting the dischargeelectrode at the center of the exhaust pipe while ensuring insulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the electric resistance-temperaturecharacteristic of a castable refractory.

FIG. 2 is the discharge voltage-current curve of a smokestack lined withcastable refractories.

FIG. 3 is a cross-sectional schematic of an electric dust collectoraccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 3 showing an embodiment, an incinerator to whichthe present invention is applied, the incinerator 2 is formed as aburning chamber on top of a building 1, and a smokestack 5, an exhaustpipe, is raised so that the smokestack is integrated with an incineratorwall 3. The smokestack 5, round in cross section, is made of steelsheeting 6 and lined with castable refractories 7. To prevent thecastable refractories 7 from falling off, many heat-resistant metallicanchors 6a are installed on the internal surface of the steel sheeting6.

The incinerator wall 3 is provided with a feed opening 9 for feedingwaste 8 in the incinerator, an air intake 10, an auxiliary burner 11,and an alkaline solution spray 12 for neutralizing gas, and theincinerator floor 4 is provided with an ash outlet 13.

A structure 15 independent of the smokestack 5 is installed on top ofthe building 1 to place a beam 16 over the smokestack 5, using thestructure. The beam 16 is electrically insulated using insulators 17. Adischarge electrode 19 is suspended from the beam 16 so that thedischarge electrode is at the center of the smokestack 5. The dischargeelectrode 19 has many needle-like discharge pins 19a almost in its lowerhalf part.

The negative pole of a DC high-voltage power supply 20 is connected withthe discharge electrode, while the positive pole is grounded andconnected with the steel sheeting 6 constituting the smokestack. Whenthe waste 8 is burned on the incinerator floor 4, high-temperatureexhaust fumes pass through the smokestack 5 and leave it at its top end.The smokestack is filled with exhaust fumes, and corona discharge occursbetween the castable refractories 7, serving as a dust collectingelectrode, and the discharge electrode 19. Dust in burnt gas is chargedby corona discharge and attracted to the internal surface, or the dustcollecting electrode, of the smokestack. Then the dust is deposited onthe castable refractories 7, which have been exposed to exhaust fumesand reached high temperature, losing the performance of electricalinsulation. As a result, the dust is neutralized. In FIG. 2, an arrowindicates the motion of the charged dust. The dust deposited on theinternal surface of the smokestack peels off under the action of its ownweight and falls along the internal surface onto the incinerator floor.

Exhaust fumes from which dust has been removed as described above areemitted at the outlet of the smokestack.

In the above embodiment, the smokestack is installed so that it isaligned with the incinerator and integrated with the wall thereof. Thepresent invention, however, is not limited to such arrangements. Theincinerator may be separated from the smokestack.

Industrial Applicability

An electric dust collector according to the present invention is usefulfor incinerators.

What is claimed is:
 1. An electric dust collector comprising:an exhaustpipe for emitting high-temperature exhaust fumes, whose internal surfaceformed from metallic sheeting is lined with refractories, a dischargeelectrode supported substantially alone the central axis of the exhaustpipe so that the discharge electrode is electrically insulated from theexhaust pipe, a high-voltage power supply for applying a high DC voltagebetween the discharge electrode and the metallic sheeting, wherein thedischarge electrode has discharge pins around, wherein the exhaust pipeis installed to be aligned with a central axis as a burning chamber andintegrated with the wall of the burning chamber, and wherein a beam isinstalled over a structure independently of the exhaust pipe so that thebeam crosses over the outlet of the exhaust pipe, and the dischargeelectrode is suspended from the beam into the exhaust pipe.